Note: Descriptions are shown in the official language in which they were submitted.
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METHOD .AND SYSTEM FOR PROVJDING THERMAL CONTROL OF
SUPER.LUMINESCENT DIODES
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not Applicable
REFERENCE TO MICROFICHE APPENDIX
[0003] Not Applicable
BACKGROUND OF THE INVENTION
[0004] The present invention relates to high intensity light sources in Fiber
Optic
Gyroscopes (referred to herein as FOGs), and more particularly, to methods of
and systems for
stabilizing, over a wide temperature range, the output characteristics of a
superluminescent diode.
[0005] FOG measurement error can generally be divided into the.categories of
bias error
and scalifzg error. All gyroscopes have a certain degree of measurement error
that is present
upon initialization, referred to as bias error. The second category of enor,
referred to herein as
scaling error (or alternatively scale factor error), accumulates over the
angle through which a
gyroscope is being rotated. Scaling error is the difference between the actual
angle of rotation
the FOG experiences and the angle of rotation indicated at the FOG output. A
FOG indicating
that it had turned ninety degrees when it had, in fact, turned ninety-two
degrees, is ari example of
scaling error. The amount of scaling error may be affected by various
environmental factors, so
that a fixed compensation value is generally not sufficient to completely
correct the FOG output.
[0006] FOGs typically use superluminescent diodes (referred to herein as SLDs)
as light
sources. The performance of a. FOG is dependent upon the wavelength of the
light source, since
the scaling error associated with the FOG is directly proportional to the
wavelength of the light
from the SLD. The wavelength of the SLD varies linearly with its operating
temperature, so it is
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necessary to temperature-stabilize the SLD with a thermoelectric cooling
module (TEC) in order
to limit scale factor variations over a wide range of FOG operating
temperatures.
[0007] Cormnercially available (prior art) SLD devices 10 typically include a
TEC
component I2 mounted internally within the SLD device package 14, as shown in
bloclt diagram
form in FIG. 1. The SLD chip 16, the light emitting element, is mounted
directly to the surface of
the TEC 12 along with a thermistor 18. The thennistor leads 20 are brought out
from the package
to permit operation with external temperature control electronics 22.
[0008] The wavelength (and consequently scale factor) sensitivity of the SLD
is typically
on the order of 400 parts per million (ppm) per degree Celcius (°C). In
order to limit scale factor
variations to within an exemplary target goal of 100 ppm, it is necessary to
control the
temperature of the SLD to within at least 0.25° C over the operating
temperature range. A
typical operating temperature range is from -54° C to 71 ° C.
The temperature control electronics
combined with an ideal TEC would be able to provide the control functions
necessary for this
level of stability. However, limitations of the construction and perfornlance
of commercially
available TEC modules preclude scale factor stability better than 100 ppm.
SUMMARY OF THE INVENTION
[0009] In one aspect, a method of stabilizing one or more output
characteristics of an
SLD device with respect to ambient temperature is disclosed. The SLD device
includes (i) a
thermoelectric cooling module (TEC) for cooling an SLD chip, and (ii) a
temperature sensor for
providing a set point signal corresponding to a set point temperature. The TEC
and the
temperature sensor, together with temperature control electronics, form a
temperature control
feedback loop for maintaining the set point temperature within a predetermined
temperature
range. The method comprises determining a variation of the one or more output
characteristics
as a function of ambient temperature. The method further includes determining
a variation of the
set point signal as a function of ambient temperature, wherein the variation
of the set point signal
corresponds to the variation of the one or more output characteuistics. The
method also includes
modifying the temperature control feedback loop so as to offset the variation
of the set point
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signal, and thereby stabilize the variation of the one or more output
characteristics as a function
of ambient temperature.
[0010] Another embodiment further includes determining a variation of an
optical power
output of the SLD device with respect to ambient temperature.
[0011] Another embodiment further includes determining a variation of a
thermistor set
point resistance, wherein the temperature sensor includes a thermistor and the
set point signal
includes a set point resistance.
[0012] Another embodiment further includes sensing a case temperature of a
case
enclosing the SLD device, and using the case temperature as ambient
temperature to modify the
temperature control feedback loop so as to offset the variation of the set
point signal. Doing so
stabilizes the variation of the one or more output characteristics as a
function of ambient
temperature.
[0013] Another embodiment further includes disposing a themnistor
substantially
adjacent to the case enclosing the SLD device, and using a resistance
associated with the
thermistor in the temperature control feedback loop, so as to offset the
variation of the set point
signal.
[0014] .Another embodiment further includes attaching the thermistor to the
case
enclosing the SLD device.
[001] Another embodiment further includes electrically coupling the thermistor
to a
bridge circuit within the temperature control electronics so as to offset the
variation of the set
point signal.
[OOI 6] Another embodiment further includes combining support circuitry with
the
thermistor in the bridge circuit such that the thermistor combined with the
support circuitry
exhibits a desired resistance profile.
[0017] .Another embodiment further includes determining ambient temperature by
sensing a case temperature of a case enclosing the SLD device.
[0018] Another embodiment further includes modifying the temperature control
feedback
loop with a processor executing code that algorithmically produces a
compensated TEC control
signal as a function of the set point signal and the ambient temperature.
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[0019] Zn another aspect, a system for stabilizing one or more output
characteristics of an
SLD device with respect to ambient temperature is disclosed. The SLD device
includes (i) a
thermoelectric cooling module (TEC) for cooling an SLD chip, and (ii) a set
point temperature
sensor for providing a set point signal corresponding to a set point
temperatur e. The TEC and
the temperature sensor, together with temperature control electronics, form a
temperature control
feedback loop fox maintaining the set point temperature within a predetermined
temperature
range. The system comprises an ambient temperature sensor disposed
substantially adjacent to a
case enclosing the SLD device, for providing a sensing signal representative
of ambient
temperature. The system further comprises offsetting circuitry, associated
with the temperature
control feedback loop, for receiving the sensing signal representative of
ambient temperature, for
offsetting a variation of the set point signal as a function of ambient
temperature, and for
providing a compensated TEC control signal to the TEC.
[0020] In another embodiment, the set point temperature sensor includes a
thennistor,
and the set point signal includes a set point resistance.
[0021] In another embodiment, the ambient temperature sensor includes a
thermistor for
sensing a case temperature of the case enclosing the SLD device.
[0022] In another embodiment, the thermistor physically contacts the case
enclosing the
SLD device.
[0023] In another embodiment, the thermistor is attached to the case enclosing
the SLD
device.
[0024] In another embodiment, the offsetting circuitry includes conductors for
electrically
coupling the thez-mistor to a bridge circuit within the temperature control
electronics.
[0025] In another embodiment, the offsetting circuitry is further combined
with support
circuitry such that the thermistor combined with the offsetting circuitry and
the support circuitry
exhibits a desired resistance profile.
[0026] In another embodiment, the offsetting circuitry includes a processor
executing
code that algorithmically produces a compensated TEC control signal as a
function of the set
point signal and the ambient temperature.
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[0027] In another embodiment, the offsetting circuitry includes an ASIC device
for
receiving the set point signal and the sensing signal representative of
ambient temperature and
producing a compensated TEC control signal therefrom.
[002a] In another aspect, a system for stabilizing one or more output
characteristics of an
SLD device with respect to ambient temperature is disclosed. The SLD device
includes (i) a
thermoelectric cooling module (TEC) for cooling an SLD chip, and (ii) a set
point temperature
sensor for providing a set point signal corresponding to a set point
temperature. The TEC and
the temperature sensor, together with temperature control electronics, form a
temperature control
feedback loop for maintaining the set point temperature within a predetermined
temperature
range. The system comprises means for sensing ambient temperature disposed
substantially
adjacent to a case enclosing the SLD device, far providing a sensing signal
representative of
ambient temperature. The system further comprises means for receiving the
sensing signal
representative of ambient temperature and for offsetting a variation of the
set point signal as a
function of ambient temperature.
BRIEF DESCRIPTION OF DRAWINGS
[0029] The foregoing and other objects of this invention, the various features
thereof, as
well as the invention itself, may be more fully understood from the following
description, when
read together with the accompanying drawings in which:
[0030] FIG. ~ ShoWB 1n block diagram form a typical prior art SLD device;
[0031] FIG. 2 shows the optical power measured from a prior art SLD device
versus
ambient temperature, at a fixed set point temperature;
[0032] FIG. 3 shows the optical power measured from a prior art SLD device
versus
thermistor set point temperature, at a .fixed ambient temperature;
[0033] FIG. 4 a block diagram of one embodiment of a system for stabilizing an
SLD
device;
[0034] FIG. 5 shows details of one embodiment of the temperature control
electronics of
FIG. 4;
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[0035] FIG. 6 shows the optical power measured from an SLD device versus
ambient
temperature, using the embodiment shown in FIG. 4; and,
[0036] FIG. 7 shows data similar to that in FIG. 6, taken on a different date.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] FIGs. 2 and 3 show the results of two parametric tests performed on a
prior-art SLD
device. FIG. 2 shows the optical power measured from the SLD device versus
ambient
temperature, at a fixed thermistor set point temperature. The external
temperature control
electronics use the thermistor signal as feedback to control the TEC, so as to
maintain the fixed
thermistor set point temperature. FIG. 3 shows the optical power measured from
the SLD device
versus the thermistor set point temperature, at a fixed ambient temperature.
Although FIGS. 2
and 3 represent the characteristics of one specific SLD device, the general
results are
representative of many SLD devices from various manufacturers.
[0038] FIG. 3 shows that the optical power output of the SLD chip is inversely
proportional to the temperature of the SLD chip, i.e., the power output
decreases linearly as a
function of increasing SLD chip temperature. FIG. 2 shows that the optical
power output of the
SLD chip is directly proportional to the ambient temperature. Considering the
results of FIG. 3,
FIG. 2 implies that the temperature of the SLD chip decreases as the ambient
temperature
increases. However, since the conditions in FIG. 2 include a fixed thermistor
set point
temperature, the actual temperature of the SLD chip can decrease only if there
is a temperature
gradient from the SLD chip to the thermistor. This phenomenon can not be the
result of heat
flow/Ieakage into the SLD device package, because during the cooling mode,
such heat flow
would tend to increase operating temperatures of the SLD chip. Local heating
of the thernzistor
would produce additional cooling to balance the heat flow resulting in a Lower
operating
temperature and hence an increase in optical power. But it is unlikely that
local heating could
result due to the small surface area of the thermistor and the low thermal
resistance to the TEC
surface. It is theorized that this phenomenon is a result of three effects:
spatial separation
between the SLD chip and the thermistor, finite in-plane conductivity of the
TEC substrate and
the temperature dependence of the thermoelectric materials.
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The equation for thermoelectric cooling is given by:
Q=aTl-'~~212R-KL~T (1)
where;
a is the Seebeck Coefficient
R is the module average electrical resistance
I~ is the module average thermal conductance
I is the electrical current
T is the mean temperature of the TEC
T is the temperature difference across the TEC
[0039] Equation (1) shows that the heat load that can be pumped across a
temperature
difference ~T is limited by (i) heat conduction through the TEC and (ii) the
joulean heat
developed because of the electrical resistance of the TEC elements. Equation
(1 ) also shows that
because the Seebeck coefficient, thermal conductance and electrical resistance
are dependent
upon the mean temperature, the cooling performance of the TEC is dependent
upon the operating
temperature.
[0040] When the SLD chip is inactive (i.e., no input power applied), the
temperature
difference across the TEC is a maximtnn for any given temperature, and the
temperature across
the TEC substrate surface is uniform. When the SLD chip is active (i.e.,
powered), the SLD chip
applies a localized heat load to the TEC, and a temperature gradient develops
across the spatial
separation between the SLD chip and the thennistor that provides feedback
control. The
temperature gradient occurs because the heat source (i.e., the SLD chip) is
localized and
conduction across the substrate is finite, limited by the thermal conductivity
of the substrate
material. Further, because the TEC materials are temperature dependent, the
temperature
difference between the SLD and the thennistor is likewise temperature
dependent.
[0041] Relationships exist between (i) the optical power of the SLD output and
the SLD
chip temperature, and (ii) the wavelength of the SLD output and SLD chip
temperature. Testing
of SLDs, exemplified by the results of FIGS. 2 and 3 were therefore used to
deduce that the
wavelength of the SLD output is changing approximately 2-3 ppm/°C case
temperature,
depending on SLD manufacturer. These wavelength sensitivity results have been
substantiated
by direct measurement of the SLD wavelength using an Optical Spectrum Analyzer
(OSA). This
implies that measures taken to stabilize the optical power sensitivity of the
SLD with respect to
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ambient temperature will also stabilize the wavelength sensitivity of the SLD
with respect to
ambient temperature.
[0042] Accordingly, the following steps provide a method of stabilizing the
wavelength
sensitivity of an SLD device with respect to ambient temperature:
1. Empirically determine the optical power sensitivity of the SLD device with
respect to
the device case (i.e., package) temperature (dP°/dT°), similar
to what is shown in FIG. 2;
2. Calculate a thennistor set point resistance change versus case temperature
(dR/dT~),
corresponding to the dP°/dT~; and,
3. Insert compensation into the temperature control feedback loop consisting
of the TEC,
the thermistor and the external temperature control electronics, where the
compensation
corresponds to an amount equal and opposite to dR/dT~.
[0043] In this description, the "ambient" temperature is determined by sensing
the
temperature of the device case. It should be noted that although the case
temperature and the
ambient temperature are related, in some cases there may be some disparity.
The disparity is
typically negligible. In some embodiments, however, the method may include
sensing true
ambient temperature. The particular SLD characterized by FIGS. 2 and 3 may be
used in the
following example to illustrate this method. FIG. 2 shows that the
dP~/dT° for this particular
SLD is approximately 0.0023 mW/°C (case). FIG. 3 shows that the optical
power sensitivity of
the SLD device with respect to the thermistor set point temperature
dP°/dTs is approximately -
0.314 mW/°C (set point). Dividing dP°/dT~ by dP°/dTs
gives the incremental change in the set
point temperature with respect to the change in the case temperature, dTSIdT~,
of -0.0073. From
data sheets associated with the thennistor, the change in thennistor
resistance with respect to set
point temperature, dR/dTs is given by 500 S2/°C. Multiplying dR/dTs by
dTSIdT~ gives:
dR/dT~ _ (500 S2/°C)(-0.0073) _ -3.65 S2/°C
[0044] The SLD represented by the data in FIGs. 2 and 3 therefore exhibits
optical power
sensitivity, with respect to case temperature, of approximately 0.0023
mW/°C (case). This power
sensitivity may be represented by a change in set point thennistor resistance,
with respect to case
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temperature, of -3.65 S?/°C. To mitigate the optical power variations
(and consequently the
wavelength variations) with respect to case temperature, the temperature
control electronics are
modified in a way that offsets the dR/dT~ variation of -3.65 SZ/°C by
an amount equal and
opposite, i.e., by+3.65 S2/°C. In one embodiment of a system 100 for
stabilizing an SLD device
102, shown in FIG. 4, the set point thermistor 104 located near the SLD chip
105 is electrically
coupled to the temperature control electronics 106, to be used as one element
in a bridge circuit
I08. The output of the bridge circuit 108 provides a control signal 110 to a
TEC drive circuit
I I2 that drives the TEC I I4 to cool the SLD chip 105. The driving circuit 1
I2 produces a
compensated TEG control signal that 113 that defines the cooling
characteristics of the TEC 114.
The set point thermistor 104, the temperature control electronics I06 and the
TEC 114 thus form
a temperature control feedback loop. In this embodiment, a case sensing
thermistor 116 is
attached to the case 1 I8 of the SLD device 102. The case sensing thermistor I
I6 is electrically
coupled to the bridge circuit 108 in a manner that offsets the variations
(with respect to case
temperature) of the set point thermistor 104. In one embodiment, the second
ihermistor 116 is
combined with additional support circuitry (not shown) so that the resulting
combination will
exhibit the desired resistance profile as a function of case temperature,
i.e., a resistance profile
that offsets the variations (with respect to case temperature) of the set
point thermistor 104.
[0045 In other embodiments, the change in set point thermistor resistance,
with respect to
case temperature (i.e., dRldT~) may be offset by other techniques known in the
art. For example,
the temperature control electronics may include an applications specific
integrated circuit (ASIC)
that produces a compensated TEC control signal as a function of the set point
temperature and
the case temperature. In other embodiments, the temperature control
electronics may include a
processor executing code (i.e., software, firmware, etc) that algorithmically
produces a
compensated TEC control signal as a function of the set point temperature and
the case
temperature. Such digitally based systems utilize A/D converters and D/A
converters known in
the au to provide the necessary interfaces between analog and digital
components. In other
embodiments, the temperature control electronics may include a look-up table
(LUT) specifically
formulated to satisfy the specif c characteristics of a particular SLD device.
Although the
embodiments generally described herein utilize thermistors for sensing set
point temperature and
case temperatures, other temperature sensing devices may also be used, such as
resistance
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temperature detectors (RTDs) other than thermistors, thermocouples, infrared
temperature
detectors, and other temperature sensing devices known in the art. In such
embodiments, the
temperature sensing device provides a temperature sensing signal, not
necessarily a resistance, to
provide au indication of the associated temperature. In other embodiments, the
temperature
sensing device may sense true ambient temperature rather than the case
temperature to provide a
signal to offset the temperature set point within the SLD device.
[0046] FIG. 5 shows details of one embodiment of the temperature control
electronics 106
of FIG. 4. The bridge circuit 108 is shown electrically coupled to the case
thermistor 116 and the
set point thermistor 104. The bridge circuit 108 provides a control signal 110
to the TEC drive
circuit 112, which drives the TEC 114 as described herein.
[0047] FIGs 6 and 7 illustrate optical power data taken on two different
dates,
coixesponding to the SLD device associated with FIGs. 2 and 3, using the case
temperature
feedback embodiment described in FIG. 4. The data of FIGs. 6 and 7 indicates
that this
embodiment provides about a ten-fold improvement in temperature and hence
scale Factor
stability.
[0048] Based upon the concepts and embodiments described herein, alld assuming
a
reasonable level of process control by the SLD manufacturer, a single
embodiment encompassing
a particular compensation scheme would be applicable to all devices from a
single manufacturer
to satisfy a moderate range of scale factor stability requirements. In order
to satisfy unusually
stringent scale factor stability requirements, tailoring the compensation
scheme for each
individual SLD device would provide a higher degree of scale factor stability.
[0049] The invention may be embodied in other specific forms without departing
from the
spirit or essential characteristics thereof. The present embodiments are
therefore to be
considered in respects as illustrative and not restrictive, the scope of the
invention being
indicated by the appended claims rather than by the foregoing description, and
all changes which
come within the meaning and range of the equivalency of the claims are
therefore intended to be
embraced therein.
